?> If you get errors about `PORTB/DDRB`, etc not being defined, you'll need to convert the keyboard's code to use the [GPIO Controls](gpio_control.md) that will work for both ARM and AVR. This shouldn't affect the AVR builds at all.
To ensure compatibility, provide validation, and power future workflows, a keyboard should declare its `pin compatibility`. For legacy reasons, this is currently assumed to be `promicro`.
Currently the following pin compatibility interfaces are defined:
If a board currently supported in QMK uses a [Pro Micro](https://www.sparkfun.com/products/12640) (or compatible board), the supported alternative controllers are:
The Proton C only has one on-board LED (C13), and by default, the TXLED (D5) is mapped to it. If you want the RXLED (B0) mapped to it instead, add this line to your `config.h`:
| [Backlight](feature_backlight.md) | Forces [task driven PWM](feature_backlight.md#software-pwm-driver) until ARM can provide automatic configuration |
| USB Host (e.g. USB-USB converter) | Not supported (USB host code is AVR specific and is not currently supported on ARM) |
| [Split keyboards](feature_split_keyboard.md) | Partial - heavily dependent on enabled features |
### Adafruit KB2040 :id=kb2040
The following defaults are based on what has been implemented for [RP2040](platformdev_rp2040.md) boards.
| [RGB Lighting](feature_rgblight.md) | Enabled via `PIO` vendor driver |
| [Backlight](feature_backlight.md) | Forces [task driven PWM](feature_backlight.md#software-pwm-driver) until ARM can provide automatic configuration |
| USB Host (e.g. USB-USB converter) | Not supported (USB host code is AVR specific and is not currently supported on ARM) |
| [Split keyboards](feature_split_keyboard.md) | Partial via `PIO` vendor driver - heavily dependent on enabled features |
The Bonsai C4 only has one on-board LED (B2), and by default, both the Pro Micro TXLED (D5) and RXLED (B0) are mapped to it. If you want only one of them mapped, you can undefine one and redefine it to another pin by adding these line to your `config.h`:
```c
#undef B0
// If Vbus detection is unused, we can send RXLED to the Vbus detect pin instead
No peripherals are enabled by default at this time, but example code to enable SPI, I2C, PWM, and Serial communications can be found [here](/keyboards/custommk/bonsai_c4_template).
## Elite-C
If a board currently supported in QMK uses an [Elite-C](https://keeb.io/products/elite-c-low-profile-version-usb-c-pro-micro-replacement-atmega32u4), the supported alternative controllers are:
